Surface finish and dimension controlled and parallel faceted semiconductor crystals and growing process



United States Patent O 3,547,708 SURFACE FINISH AND DIMENSION CONTROLLED AND PARALLEL FACETED SEMICONDUCTOR CRYSTALS AND GROWING PROCESS Thomas H. Strudwick, Wappingers Falls, N.Y., assignor to International Business Machines Corporation, Armonk, N.Y., a corporation of New York No Drawing. Filed Jan. 13, 1967, Ser. No. 608,979 Int. Cl. B01j 17/18; C01b 33/00 US. Cl. 148--1.6 3 Claims ABSTRACT OF THE DISCLOSURE Single non-dendritic crystals of diamond cubic crystal lattice semiconductor materials have blemish free and substantially atomically flat parallel natural oriented faces which extend longitudinally along opposite sides of a non-dendritically grown crystal. The faces are parallel to each other and parallel to the direction of growth, and the non-dendritic crystals have controlled thicknesses between opposite parallel faces. Also described herein are non-dendritic processes for growing non-dendritic single crystals having flat and blemish free faces. Crystals grown according to this invention may be used without further refinement for the production of semiconductor devices. 1

BACKGROUND OF THE INVENTION Single crystals of diamond cubic crystal lattice semiconductor materials having blemish free and substantially atomically flat natural oriented facets 180 apart, extending longitudinally along opposite sides of a grown crystal parallel to each other and parallel to the direction of growth and having controlled thickness, and processes for growing non-dendritic single crystals having fiat and blemish free faces, which may be used without surface refinement for the production of semiconductor devices, are disclosed herein.

Crystal growing processes for crystals of semiconductor materials exhibiting diamond cubic or zinc blende lattices are well known. Two of the most important semiconductor materials which are used in those crystal growing processes are silicon and germanium.

In one of the well known processes, seed crystals are inserted into a molten mass of semiconductor material, and as the seed begins to melt, the entire mass is supercooled. The seed crystal, being the only or the most prominent nucleation center, the material begins to solidify on the seed; at the same time the seed is slowly withdrawn from the pool of material, growing and pulling with it an elongated dendrite. This process is conventionally denoted as a Controlled Dendritic Growth Process, but for simplicity it is referred to herein as a Dendritic Process.

In another crystal growing process, seed crystals are brought into contact with molten material at a temperature slightly above the melting point and then are drawn away at slow rates of speed. Usually the melt is not supercooled as in the former process, but there exists slight depressions in temperature below the freezing point in the solidifying front of a growing crystal. For convenience, this latter process is hereinafter identified as a nondendritic process.

Crystals with fiat faces have been grown in dendritic processes. However, certain drawbacks which are inherent in the dendritic process result in inferior crystals of unpredictable quality, which often must be surface finished before use in semiconductor devices. A theory of dendritic growth proposed by Hamilton and Seidensticker requires two or more twin planes to prevent the 3,547,708 Patented Dec. 15, 1970 elimination of re-entrant corners which appear to be necessary during crystal formation for continued nucleation and growth of dendrites. Due to their growth mechanism dendrites are subject to violent segregation of impurities and are somewhat difiicult to control because of their growth from supercooled melts. It has been difficult to produce blemish free, flat and parallel faces with dendritic process. Moreover, problems exist-in precisely controlling dendrite and web thickness.

SUMMARY OF THE INVENTION The present invention comprises a non-dendritic crystal growing process used for the growth of parallel-faceted monocrystals of materials exhibiting a diamond cubic structure, notably but not exclusively silicon and germanium. The process incorporates the utilization of a seed crystal, exhibiting any odd number of coherent parallel twin planes oriented perpendicular to a 1l1 direction and parallel to the direction of growth for initiating controlled nucleation and growth from a melt of a monocrystalline material. The grown monocrystal exhibits a continuation of the twin planes of the seed plus two natural (111) faces on the grown crystals periphery. These faces are mutually parallel to the twin planes and to each other. The faces appear to be a result of the implementation of a particular seed in such a manner as to induce nucleation and growth of a desired monocrystal in a crystallographic direction opposite to that depicted as the [213] direction in the Journal of Applied Physics, volume 31, No. 7, July 1960, page 1166, FIG. 2.

This invention has among its objectives the provision of a non-dendritic grown semiconductor monocrystal ribbon having blemish free, substantially atomically flat and controllably spaced opposite parallel faces extending in a direction of crystal growth. A non-dendritic process for growing such crystals is another object of the invention. One purpose of the process is to produce semiconductor crystal strips having dimensions and topography suitable for immediate use or subsequent oxidation, epitaxial growth, diffusion or other processes for producing semiconductor devices. The semi-continuous nondendritic growth of crystalline ribbons for semiconductor device fabrication and the production of windows for the distortion-free transmission of coherent or non-coherent infra-red light are two possible products of the described process.

The foregoing and other objects, features and advantages of the invention will be apparent from the followingmore particular description of a preferred embodiment of the invention.

DETAILED DESCRIPTION In a preferred embodiment of the process of the present invention a seed crystal is cut from a larger crystal of a semiconductor material, so that the seed contains one or any other odd number of twin planes. The seed crystal is oriented with respect to the surface of a molten pool of semiconductor material, which is maintained at a temperature slightly above the melting point of the material. The seed is fastened to a reciprocally movable holder which allows the seed to move toward and away from the melt surface along a crystallographic 21l direction, while the seed is mounted with its [511] direction towards the surface of the melt. The seed is moved toward the melt, and as the melt is contacted] by the seed crystal, a portion of the seed is liquified. Thereupon the seed crystal is drawn away from the melt, and the grown continuation of the crystal freezes at a solidification front having a temperature slightly depressed from the freezing point of the material. As the crystal continues to solidify and grow, the twin plane or planes of the seed crystal are continued,v

in the growing portion, and these twin planes cause the characteristic blemish free and substantially atomically flat facets that are an objective of this invention. Moreover, the constant temperatures in the process and the existence of a twin plane or of an odd number of twin planes in the grown crystal have a stabilizing effect upon its thickness.

In an early test two silicon crystals were grown using the techniques described above. Examination of the crystals revealed odd numbers of twin planes. Later examination using extinction contrast X-ray microscopy disclosed the crystals to be practically free of dislocations. Several attempts were made to grow dual-faceted crystals in the [Zfi] crystallographic direction of seed crystals; however, facet formation was never observed.

In one example, seed was cut from a crystal which had existing multiple odd number twin planes. Using a NRC puller type furnace and a two-inch round bottom crucible holding Dow Corning polycrystalline silicon and with an argon atmosphere flowing through the furnace at six liters per minute, a crystal was grown according to the above conditions. The furnace was cleaned and the seed was mounted in the prescribed manner. A one-tenth millimeter vacuum was pumped on the chamber, which was then back-filled with argon. The pull rate was three inches per hour; the seed rotation rate was thirty revolutions per minute. The crucible lift rate was about one inch per hour, and the rotation rate of the crucible was five revolutions per minute. The resulting boule was oriented on a 211- axis; the direction of crystal growth was [21 1]. No external facets were apparent. The multiple odd numbered twin planes of the seed were found to be carried through the entire crystal.

' In a second example the same seed was used with all conditions identical, except that the seed was inverted so that the crystal growth occurred in the [ill] direction. Flat external natural facets were observed parallel to each other and to the twin planes.

In another example with the furnace cleaned and conditions and procedure the same as for the previous example, an attempt was made to produce a crystal with only one twin plane. A new elongated three-sixteenths inch square cross section seed was cut from a crystal which under microscopic examination was determined to have only a single twin plane. Following the previously outlined steps a crystal was grown in the [511] direction. A test slice was removed from the boule at the tail end remote from the seed. Before examining the slice under a microscope both surfaces Were lapped with silicon carbide lapping compound and were mechanically polished on microcloth before etching and studying. It was determined that there was only one single twin plane throughout the entire length of the crystal.

While the invention has been particularly described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. The process for melt-growing non-dendritic crystals of semiconductor material comprising maintaining a molten semiconductor material melt at a temperature at least as great as the melting point of the material, orienting a seed crystal above a surface of the melt, with a [51 1] direction of the seed crystal pointed toward the surface of the melt, the seed crystal having an odd number of twin planes oriented perpendicular to the surface of the melt, contacting the molten semiconductor material with the seed crystal, uniformly drawing the seed crystal away from the molten material, thereby creating a ribbon, cooling the ribbon to a temperature below the freezing point of the semiconductor material, and solidifying the material at a solidifying front in the ribbon at a point spaced from the surface of the material, thereby creating a crystal having an odd number of twin planes related to the odd number of the twin planes of the seed crystal and having atomically flat surfaces parallel to the twin planes.

2. The process for melt-growing non-dendritic diamond cubic lattice semiconductor crystals having two parallel, blemish-free, and substantially atomically flat natural (111) facets forming opposite parallel crystal sides which are parallel to crystal growth direction, the parallel facets being uniformly spaced apart at a controlled distance, comprising the steps of maintaining a melt of semiconductor material at a temperature at least as high as the melting point of the material, pointing a seed crystal in a [511] direction toward a surface of the melt, the seed crystal having an odd number of twin planes perpendicular to a (111) direction and perpendicular to the surface of the melt; constraining the seed crystal for reciprocal movement with respect to the melt in a 211 direction; moving the seed crystal in a [211] direction int 0 contact with the melt, uniformly withdrawing the seed crystal from the melt in a [Zfi] direction, thereby forming a ribbon of semiconductor material, cooling the ribbon of semiconductor material at least to its freezing point and freezing the material at an area in the ribbon spaced from the surface of the melt, thereby growing a single crystal ribbon in a [i1 1 1 direction, the crystal having an odd number of twin planes oriented perpendicular to the surface of the melt and parallel to the direction of growth, and having opposite, parallel natural (111) facets extending along the outside of the ribbon in the direction of crystal growth.

3. A non-dendritic melt-grown crystal of a semiconductor material crystallizing in the diamond cubic lattice structure having two relatively flat parallel faces comprised of (111) planes, and having a single interior twin plane extending entirely therethrough and relatively parallel to the parallel faces.

Longini et al.: Growth of Atomically Flat Surfaces on Germanium Dendrites, Journal of Applied Physics, vol. 31, No. 2, July 1960, Pp. 1204-1207.

EARL C. THOMAS, Primary Examiner H. S. MILLER, Assistant Examiner US. Cl. X.R. 23204, 301 

